1. Field of the Invention
The present invention is directed to imaging spectrometers, and more particularly to scalable imaging spectrometers.
2. Description of the Related Art
A spectrometer measures the spectrum, or wavelength distribution, of a particular light beam. The light beam may be generated by a light source, or may be reflected from a sample or object. In one typical application, a spectrometer is flown in an aircraft and analyzes the light reflected from a distant object on the ground. The incident light beam, which is essentially collimated upon entering the spectrometer, strikes a dispersive optical element such as a diffraction grating or prism, which redirects each spectral component of the beam with a slightly different propagation angle. The spectrally divergent beam is then focused onto a one-dimensional array of detector elements or pixels, with the output from each pixel corresponding to an optical power in a particular spectral range.
Spectrometers are also useful in near-field applications. A typical near-field application is the fluorescence imaging of a material sample. The sample may be illuminated externally, or may not require any external illumination. Regardless of how the sample is lit, light from the sample is collimated before it strikes the dispersive optical element, and continues to pass through the spectrometer in essentially the same manner as described above.
These spectrometers that use one-dimensional arrays generally cannot capture any spatial information about the particular light beam and therefore generate only spectral information.
A class of spectrometers known as imaging spectrometers, or hyperspectral imagers, has been developed that uses a two-dimensional array of detector elements or pixels, such as a CCD array, rather than a one-dimensional array. An important subset of these imaging spectrometers are so-called “push-broom” imaging spectrometers, which image a long, thin section of the object of interest off a dispersive element, such as a diffraction grating, onto a two-dimensional focal plane array. This approach provides a detailed spectrum for each “pixel” of the long, thin region per frame. The spatial information in the second dimension is then obtained by scanning the array across the object and correlating subsequent one-dimensional images obtained by the CCD army over a period of time. Note that data in three dimensions is thus obtained by a scanned push-broom imaging spectrometer—two spatial dimensions of an object plus the spectrum for each recorded pixel.
When a push-broom imaging spectrometer is designed for use in an aircraft, scanning is accomplished typically by flying the imaging spectrometer over the object. Given the heights at which aircraft typically fly (greater than say, 100 meters), the light reflected from an object on the ground may be considered essentially collimated. Light rays leaving a point on the object on the ground are, therefore, essentially parallel upon reaching the imaging spectrometer as it flies overhead.
In a typical push-broom imaging spectrometer, an objective lens or mirror brings the essentially parallel rays to a focus, forming an internal image of the distant object. The internal image is located on a slit, which transmits only a narrow portion of the image and blocks everything else. Note that in general, an aperture located at an internal image acts as a field stop for the optical system, allowing only rays from certain portions of the object to pass through the system. For the push-broom imaging spectrometer, the slit functions as a field stop, allowing rays from only a thin slice of the object to pass through the spectrometer at any one time. The slit is generally oriented perpendicular to the scan direction, i.e., the direction in which the plane flies.
After passing through the slit, the light diverges and strikes a curved diffraction grating, which focuses the light onto a two-dimensional array of detector elements, such as a CCD camera. Alternatively, the light diverging from the slit first is collimated by a lens or mirror, then strikes a flat or curved diffraction grating, then is focused by a lens or mirror onto a two-dimensional array of detector elements, such as a CCD camera. In the alternate arrangements, the multiple reflections or lenses may allow for flattening of the optical field at the CCD camera, allowing for sharp imaging at both the center and the edges of the CCD pixel array.
The grating lines are generally oriented parallel to the slit, and the grating is generally optimized to maximize its output into a particular diffracted order. At the CCD array, the spectral information is obtained along an axis perpendicular to the grating lines, and the spatial information is obtained along an axis parallel to the grating lines.
In most imaging spectrometers to date, a rotationally symmetric focusing optic is used on both sides of the slit. Consequently, the beam is brought to a focus at the slit in both the spectral and spatial dimensions. For example, a device commonly known as an Offner imaging spectrometer is disclosed in U.S. Pat. No. 5,880,834, issued Mar. 9, 1999 to Michael P. Chrisp, and utilizes rotationally symmetric focusing optics on both sides of the slit. An example of a predominantly lens-based imaging spectrometer is disclosed in U.S. Pat. No. 6,734,966, issued May 11, 2004 to James K. McCarthy. An example of a more traditional mirror-based spectrometer is disclosed in U.S. Pat. No. 5,305,082, issued Apr. 19, 1994 to Georges G. Bret. Other examples using simpler technology are disclosed in U.S. Pat. No. 2,922,331, issued Jan. 26, 1960 to William G. Fastie et al., and U.S. Pat. No. 4,743,112, issued May 10, 1988 to Elliot M. Burke.
In the spectral dimension, the field of view is kept relatively small so that the range of incident angles on the grating is tightly constrained, and this relatively small field of view effectively eliminates the off-axis aberrations in the spectral dimension.
In the spatial dimension, where the field of view does not significantly affect the spectral resolution of the grating, the field of view is increased significantly beyond the spectral dimension. In fact, in many cases it is desirable to have a large field of view in the spatial dimension, so that when the spectrometer is flown overhead, a large portion of the ground may be imaged by a single overhead pass of the aircraft. As the field of view increases, however, off-axis aberrations, both in the focusing optic and in the downstream spectral dispersive module of the system, generate increasingly large image blur and limit the performance of the spectrometer.
Because the spectrometer is typically flown in an aircraft, its size is important. For example, use in a small, uninhabited aerial vehicle (UAV) may require that the spectrometer be scaled down to a fraction of its typical size. Such scaling poses a variety of problems in performance.
For instance, if all the components of a spectrometer are proportionately reduced in size by a factor of 3, then the amount of light reaching the detector, or throughput, is reduced by a factor of 9. In general, such a reduction in throughput unacceptably reduces the signal-to-noise ratio of the detectors. Therefore, as a practical matter in reducing the size of the spectrometer, some components are preferably shrunk more than others, rather than reducing the size of everything in proportion.
One condition that maintains an acceptable throughput is that the entrance pupil diameter is left constant while the focal lengths of the optical elements are reduced, along with the distances between components. This results in a higher numerical aperture (NA) and, equivalently, a lower F-number (f/#). This increased NA increases the off-axis aberrations of the system, most notably at the edge of the field of view. Although one might be tempted to therefore reduce the field of view to compensate for the increase in NA, in order to reduce the off-axis aberrations in the system, in general this is unacceptable, as it is a compromise in performance.
In other words, the problem encountered with scaling down a spectrometer design is as follows. The field of view should not be scaled down, as it is generally a design specification. The entrance pupil diameter should not be scaled down either, as it determines the throughput of the system. The focal lengths of and distances between optical elements should be scaled down, resulting in a higher NA, and therefore an increased sensitivity to off-axis and other aberrations.
Hence, there exists a need for an imaging spectrometer with reduced off-axis aberrations, which would allow for its design to be scaled down without sacrificing either field of view or throughput.